How can I find what my total elevation gain is on the odometer? I can't seem to find it.

It's not one of the standard data fields. If you want to view that information on the trip computer screen, you'll need to change one of the fields to show total elevation ascent.

From the trip computer screen, press the Menu button. In the menu options, should be something like a modify data fields option - select it. After that, use the joystick to move to the field you will change, press down/enter on the joystick and select the new field from the list (there are several pages of fields available). Save changes and exit. The new elevation field should now display on the trip computer screen.

Just play around, I think there is even a menu option to reset the trip computer back to default data fields? If all else fails, check around pages 28 and 47 of your etrex user manual.